The term magnetoencephalography (often, abbreviated by the acronym “MEG”) refers to the detection and measurement of the magnetic fields, which are produced by the electric currents, which flow naturally within the bodies of humans and animals. For example, such electric current flows are a fundamental feature of the functioning of the neurological system of a human being. Charged ionic flows within the neurons, which make up part of the human brain are, in effect, an electric current, which, produces a magnetic field, which can be measured using the methods of MEG. The electric currents, which drive the pumping of the heart in an animal, produce magnetic fields, which can be measured using magnetocardiography. Measurements of the magnetic fields produced by these electric currents can be used to deduce information about the size and direction of the currents as a function of time as well as their location and distribution within the body of a person, and therefore to provide information about the state of health and the state of function of the person.
Apparatus and methods of MEG have been developed and expanded over the past forty years, enhancing sensitivity to enable the detection of magnetic fields produced by electric currents flowing deep within the body. The “field of view” of the magnetometers used for MEG have been systematically expanded from single channel detectors of the magnetic field at one location to large helmet-shaped systems measuring the values of the magnetic fields at up to 275 locations around the head of a human being or up to 150 locations over the chest of a human being.
Magnetoencephalography has also been used to measure magnetic fields produced by electric currents flowing in biologic samples such as brain tissue slices of laboratory animals. In these systems, methods have been developed to bring the detector of the magnetic field as close as possible to the electric current itself to maximize the size of the measured signal and the ratio of the signal to the background magnetic noise. In some case, spacing as small as 1 mm or less has been achieved.
Generally, the biomagnetic measurements of biogenic electric currents are useful for measuring the distribution of such currents in a tissue such as a brain slice or in an organ such as a brain or heart.
One major limitation in the application of the biomagnetic techniques for the purposes outlined above arises from a fundamental property of magnetic fields produced by electric currents flowing in tissues or organs. Any such tissues or organs can be described by a circuit of electrically active cells that produce the biogenic current. In intact humans or animals the tissue of the organ that contains such electrogenic cells is saturated with physiological saline. In in vitro preparations, such a tissue is immersed in a bath of physiological saline. The physiological saline conducts electricity; thus, the medium containing the saline such as the brain or the head, or a bath containing the tissue, is called “conductive medium.” From the fundamental principles governing electromagnetism in conductive media, an electric current which flows within and proximate to the surface of such a conductive medium and flows in a direction which is perpendicular to the surface of that medium produces no net magnetic field external to the medium itself. This is strictly true when the conducting medium is spherical or flat. A large bath can be thought of as a part of an infinitely large sphere. But, it is very well approximated even in a conducting medium that lacks a spherical symmetry when the cells are close to the boundary separating the conducting medium from the surrounding non-conducting medium. This factor has limited the utility of biomagnetic measurements such as MEG in providing complete information about electric currents in a tissue or in an organ. The conventional biomagnetic techniques can provide the information only about those components of the electric currents flowing within conducting media, which flow in a direction parallel to the surface of that medium, but not the currents, which flow normal to the surface. In particular, this factor has impacted heavily on the application of MEG to examine the brains of prematurely born human babies, since in these babies the cerebral cortex is poorly developed and larger percentages of neuronal activity are perpendicular to the surface of the skull and cannot be easily detected with conventional biomagnetic techniques. In general, this factor has significantly constrained the application of the biomagnetic techniques for measuring biogenic currents from human and animal brains.
The term transcranial magnetic stimulation (often abbreviated by the acronym “TMS”) refers to the process of applying a pulse of magnetic field to the brain of an animal or human being in order to stimulate the neurons within the brain. The technology of TMS is now well known and the procedure of TMS is routinely used for both research and clinical purposes. A summary of current methods is given in chapter 22 of the text Bioelectromagnetism, authored by J. Malmivuo and R Plonsey, published by the Oxford University Press in 1995.
All of the current TMS methods utilize electrical coils placed on or adjacent to the head of the subject to produce a magnetic field within the brain when a pulse of electrical current is sent through the coils. Generally, TMS coils are constructed in a planar format and placed as close to the head as practical to maximize the strength of the magnetic field within the head, and hence the corresponding induced stimulating electric field. The pulsed magnetic field produced by such coils typically has a direction, which is perpendicular to the plane of the coils and to the adjacent surface of the head. This pulsed field then induces a pulsed electric field within the head and within the brain, which is oriented orthogonal to the direction of the pulsed magnetic field. The pulsed electric field then stimulates primarily those neurons within the structure of the brain, which are parallel to the induced electric field.
The brain comprises a complex structure with many folds and convex as well as concave surfaces. The cortex of the brain contains large numbers of pyramidal neurons, which line the surfaces. An electric field applied in a direction parallel to a neuron, if sufficiently strong, can cause that neuron to ‘fire’ or activate. However, the same electric field applied in a direction orthogonal to a neuron will generally not result in activation. Thus the current methods and apparatus used for TMS of the brain only stimulate a portion of all of the neurons within the brain, namely the neurons with an orientation parallel to the nearby surface of the head. There is a need to find a way to stimulate those neurons, which are oriented perpendicular to the surface of the head